Friendly Numbers
Add up a number's divisors, divide by the number itself, and you get its "abundancy." Take 30: its divisors are 1, 2, 3, 5, 6, 10, 15, and 30, which sum to 72 — and 72 divided by 30 is 12/5.
Now take 140: its divisors are 1, 2, 4, 5, 7, 10, 14, 20, 28, 35, 70, and 140, summing to 336 — and 336 divided by 140 is also 12/5.
Two numbers with the same abundancy ratio are called a "friendly pair." 30 and 140 are one; so are 2480, 6200, and 40640, which all share that same 12/5 ratio with each other.
Based on a number-theory post on Futility Closet.